For the past 4 years I was using a local agent in Singapore to book my flights. With the downturn in the economy I have been booking myself on the SIA web site. I was listing my UA FF Number in the process. After several i had checked on United only to discover the flights were not showing for the miles flown. I contacted SIA to which they indicated because of the classification they could not award to United. They said only SIA could be used of which I had no choice but to agree. I asked why the system could not be set to reject the United FF number so the individual would then have a choice. I indicated that the web site booking process could be easily change as it is just a coding issue.